Mud Patch Research Articles

Spatial Distributions of Surface Sedimentary Organics and Sediment Profile Image Characteristics in a High-Energy Temperate Marine RiOMar: The West Gironde Mud Patch

The spatial distributions of (1) surface sediment characteristics (D0.5, Sediment Surface Area (SSA), Particulate Organic Carbon (POC), Chlorophyll-a (Chl-a), Phaeophytin-a (Phaeo-a), Total and Enzymatically Hydrolyzable Amino Acids (THAA, EHAA), δ13C) and (2) sediment profile image (apparent Redox Potential Discontinuity (aRPD), numbers and depths of biological traces) characteristics were quantified based on the sampling of 32 stations located within the West Gironde Mud Patch (Bay of Biscay, NE Atlantic) in view of (1) assessing the spatial structuration of a temperate river-dominated ocean margin located in a high-energy area, (2) disentangling the impacts of hydrodynamics and bottom trawling on this structuration, and (3) comparing the West Gironde Mud Patch with the Rhône River Prodelta (located in a low-energy area). Results support the subdivision of the West Gironde Mud Patch in a proximal and a distal part and show (1) the existence of depth gradients in surface sedimentary organics characteristics and bioturbation within the distal part; (2) no evidence for a significant effect of bottom trawling, as opposed to Bottom Shear Stress, on the West Gironde Mud Patch spatial structuration; and (3) major discrepancies between spatial structuration in the West Gironde Mud Patch and the Rhône River Prodelta, which were attributed to differences in tidal regimes, sedimentation processes, and local hydrodynamics, which is in agreement with current river-dominated ocean margin typologies.

Wave speed and attenuation profiles in a stratified marine sediment: Geo-acoustic modeling of seabed layering using the viscous grain shearing theory.

In the viscous grain shearing (VGS) theory of wave propagation in an unconsolidated sediment, the dispersion formulas for the phase speed and attenuation of the compressional and shear wave involve two parameters, the compressional modulus, γp, and the shear modulus, γs, which depend on the radius of the circle of contact between contiguous grains in the granular medium. The radius of contact itself depends on the overburden pressure, and hence depth, in the sediment. Based on these observations, the VGS theory is extended to create a geo-acoustic model of a horizontally stratified sediment in which each layer has a uniform porosity, bulk density, and mean grain size, all of which are assumed known from geological survey data. In a given layer, the overburden pressure consists of the contributions from all the higher layers. From the overburden pressure, the compressional and shear moduli are expressed as functions of depth throughout the layer, thereby allowing the frequency dependent phase speed and attenuation profiles of both types of wave to be computed from the VGS dispersion formulas. To illustrate the VGS geo-acoustic modeling technique, two examples are discussed, one of which represents the mud overlying sand sediment at the New England Mud Patch.

Biodiversidad y distribución de Gasterópoda en Playa Reina, Mariato, Veraguas, Panamá.

Este artículo es sobre las especies de Gasteropoda recolectadas en playa Reina, en el distrito de Montijo, provincia de Veraguas, depositadas en la Colección Nacional de Referencia del Museo de Malacología de la Universidad de Panamá (MUMAUP). Mariato, se ubica a 7°37’16.99” LN y 80°59’42.79” LO. La playa tiene un extenso litoral rocoso de piedras y rocas, pocas secciones arenosas con parches de fango-arena, el agua es turbia, y está llena de mucho sedimento. Los ejemplares se recolectaron manualmente de seis biotopos (rocas, piedras, fango-arena, arena, coral muerto y estero); registrándose un total de 26 324 individuos, 133 especies, 82 géneros, 42 familias, 38 superfamilias y ocho órdenes; 99 especies estuvieron en las rocas, 38 en las piedras y 15 en ambos lugares del litoral rocoso. Las rocas fueron el biotopo donde más especies se recolectaron, ninguna se encontró en más de dos biotopos. Se recolectaron ejemplares de importancia económica como Malea ringens (Swainson, 1822), Hexaplex radix (Gmelin, 1791), H. regius (Swainson 1821) y Vasula melones (Duclos 1832). Las poblaciones de caracoles que dominaron en el área fueron las adaptadas a los fuertes oleajes y las que se adhieren al biotopo duro para resistir las corrientes de las aguas marinas.

Recent coastal anthropogenic impact recorded in the Basque mud patch (southern Bay of Biscay shelf)

The historical anthropogenic impact on sediments from the Basque Mud Patch (southern Bay of Biscay) is explored using a multidisciplinary approach including the analysis of natural (210Pb) and artificial (137Cs, 239/240Pu) radiotracers, major elements (Al, Mn), metals (Zn, Pb, Cu, Cr), Pb isotopic ratios, and foraminiferal and pollen contents. The study of three short cores (19–46 cm), despite being hindered by the effects of biomixing, allow the calculation of a sedimentation rate of 1 ± 0.1 mm yr−1. Distribution with depth of Al-normalised concentrations of metals reflects an increasing trend since 1880 CE, related to the industrialization of the Basque coastal area. According to the Sediment Quality Guidelines applied, contents of Zn and Pb appear as a potential cause of concern, given that they exceed the values from which adverse biological effects can be occasionally expected. However, foraminiferal assemblages do not show recognizable changes along the cores following increasing trace metal concentrations. Finally, pollen results reveal an increasing trend of coniferous taxa and a parallel reduction of authochthonous Deciduous Quercus, probably as a consequence of reforestation works. Data obtained confirm that effects of coastal anthropogenic activities extend to the adjacent shelf, where muddy deposits are likely to act as a trap for contaminants.

Measurement of Sound Speed in Fine-Grained Sediments During the Seabed Characterization Experiment

The Seabed Characterization Experiment was carried out from March 5 to April 10, 2017 (SBCEX17) on the New England Mud Patch, approximately 90 km south of Martha's Vineyard. The SBCEX17 experimental site covers an area of 11 km × 30 km with water depth in the range of 75–80 m. The Sediment Acoustic-speed Measurement System (SAMS) is designed to measure sediment sound speed and attenuation simultaneously over the surficial 3 m of sediments. During SBCEX17, SAMS was successfully deployed at 18 sites, which were chosen to coincide with coring locations, with the goal of developing a geoacoustic model for the study area. In this article, a summary of SAMS operation during SBCEX17 is presented, as well as preliminary results for sediment sound speed and its spatial variation in the frequency band of 2–10 kHz. It is found that in mud, the sound-speed ratio is in the range of 0.98–1. Little dispersion was observed in this frequency band. Using the preliminary SAMS sound-speed results measured at different depths, the sound-speed gradient in mud within the surficial 3 m favors an exponential rather than a linear dependence at SBCEX17 site. Large gradients are observed for depth shallower than 1.5 m. For the sandy basement beneath the mud layer, the sound-speed ratio is as high as 1.105.

Modern and Fossil Pockmarks in the New England Mud Patch: Implications for Submarine Groundwater Discharge on the Middle Shelf

AbstractGeophysical evidence is presented for the existence of modern and fossil pockmarks within the New England Mud Patch, an anomalous region of fine‐grained deposition on the middle shelf south of Cape Cod, USA. Modern pockmarks, indicators of focused subsurface fluid flow, are observed as seafloor depressions up to 80‐m wide and 1.2‐m deep, with reflective centers evincing hard material. Probable fossil pockmarks are identified by predominantly u‐shaped bright lenses within the sub‐bottom data and have similar size and distribution as the seafloor pockmarks; they likely indicate long‐term (>10 ka) persistence of episodic‐focused fluid flow. Given the lack of seismic evidence for gas, these features are attributed to submarine groundwater discharge. Based on recent observations and modeling, it is suggested that submarine groundwater discharge is induced by salinity‐driven convection cells associated with overpressured fluids, emplaced either by modern, shore‐connected aquifers, or during Pleistocene glacial advances and retreats.

Trans-dimensional inversion of modal dispersion data collected by an underwater glider

Acoustic payload equipped underwater gliders have been proven to have great potential for maritime intelligence, surveillance, and reconnaissance missions, as well as oceanic environment characterization. This paper demonstrates capabilities for seabed characterization using broadband signals received on a hydrophone-equipped Teledyne Webb Research Slocum glider during the 2017 Seabed Characterization Experiment (SBCEX) conducted on the New England Mud Patch. In the experiment, a source ship maintained a fixed position while two combustive sound-source signals were emitted at a separation of about two minutes in time. The glider was programmed to follow a sawtooth-like track through the water approximately 8 km from the source. Both source and glider were in an area where water depth is around 72 m. The two transmissions were received by the glider at depths separated by about 15 m. Discrepancies in the modal dispersion structure of the received signals were observed, as expected for different reception depths. Trans-dimensional geoacoustic inversion is applied to the modal-dispersion data to study the consistency of the inversion results for signals received at different depths. The benefit of combining signal receptions at different depths in the inversion to reduce the uncertainties of the geoacoustic estimates is also addressed. [Work supported by ONR.]

On the sound speed dispersion and the frequency dependence of sound attenuation in a fine-grained sediment in the New England Mud Patch

The study on the sound speed dispersion and the frequency dependence of sound attenuation in marine sediments is of great importance in understanding the physics of sound propagation in the sea bottom. Modal dispersion curves (MDC) have only been utilized to estimate the low-frequency sound speed. At higher frequencies (i.e., above a few hundred hertz), arrival time difference for different modes generally become smaller and MDC is difficult to extract because of the cross-mode interference. This paper applies time-frequency representations (TFR) with high time-frequency resolution (e.g., optimal-kernel and center-affine-filtered TFR) to broadband acoustic signals (e.g., 31g explosive and combustive source signals) detonated along the 15-km Northwest-Southeast tracks during the Seabed Characterization Experiment in the New England Mud Patch, which has a surface fine-grained sediment layer overlying a sandy bottom. The resulting high-resolution broadband (150–1500 Hz) MDC, in conjunction with TL data, are fed to a previously developed two-step dimension-reduced geoacoustic inversion algorithm including modal dispersion-based inversion for sound speed and energy-based inversion for attenuation [IEEE J. Ocean. Eng., 44, in print, (2019)]. The estimated sound speed and attenuation are compared with the theoretical results from seabed geoacoustic models for fine-grained sediments. [Work supported by ONR Ocean Acoustics (322OA).]The study on the sound speed dispersion and the frequency dependence of sound attenuation in marine sediments is of great importance in understanding the physics of sound propagation in the sea bottom. Modal dispersion curves (MDC) have only been utilized to estimate the low-frequency sound speed. At higher frequencies (i.e., above a few hundred hertz), arrival time difference for different modes generally become smaller and MDC is difficult to extract because of the cross-mode interference. This paper applies time-frequency representations (TFR) with high time-frequency resolution (e.g., optimal-kernel and center-affine-filtered TFR) to broadband acoustic signals (e.g., 31g explosive and combustive source signals) detonated along the 15-km Northwest-Southeast tracks during the Seabed Characterization Experiment in the New England Mud Patch, which has a surface fine-grained sediment layer overlying a sandy bottom. The resulting high-resolution broadband (150–1500 Hz) MDC, in conjunction with TL data, are ...

Geoacoustic inversion using vector acoustic modal dispersion

Geoacoustic inversion is traditionally performed using sound pressure data. Here, we present a study of geoacoustic inversion using particle velocity, as recorded on a vector sensor. The scope of the study is restricted to shallow water and low-frequency impulsive sources. In this context, the propagation is described by normal mode theory. The modal description of the vector field is well known: the mode amplitudes are different than from the pressure field, but their phase is the same. As a result, modes can be filtered from particle velocity data in the same way that they are filtered from pressure data, as long as one considers filtering methods that are based on the modal phase. We show that warping (a non-linear resampling scheme) enables modal filtering on both pressure and particle motion data. Then, the filtered modes (pressure and velocity) are coherently combined to form four vector acoustic metrics [Dahl and Dall’Osto, IEEE J. Oceanic Eng. 2019]. Each metric is used as an input for Bayesian geoacoustic inversion. The whole method is illustrated on a simulated scenario that reproduces the Seabed Characterization Experiment (SBCEX) that took place on the New England Mud Patch in 2017. [Work supported by the Office of Naval Research.]

Acoustical oceanography with a single hydrophone: Propagation, physics-based processing and applications

Lobsters, whales and submarines have little in common. Except that they produce low-frequency sound, like many other marine occupants that use sound for communication, foraging, navigation and other purposes. However, unraveling and using the underwater cacophony is not at all simple. This is particularly true for low-frequency (f < 500 Hz) propagation in coastal water (water depth D < 200 m), because the environment acts as a dispersive waveguide: the acoustic field is described by a set of modes that propagate with frequency-dependent speeds. In this context, to extract relevant information from acoustic recording, one needs to understand the propagation and to use physics-based processing. In this tutorial-like presentation, we will show how to analyze low-frequency data recorded on a single hydrophone. We will notably review modal propagation and time-frequency analysis. We will then show how those can be combined into a non-linear signal processing method dedicated to extract modal information from single receiver, and how such information can be used to localize sound sources and/or characterize the oceanic environment. The whole method will be illustrated on several experimental examples, including geoacoustic inversion on the New England Mud Patch and baleen whale localization in the Arctic.

Investigating gradients in mud based on Bayesian modal-dispersion inversion and a hybrid geoacoustic model parameterization

An important issue in understanding seabed geoacoustic properties at the site of the 2017 Seabed Characterization Experiment (SBCEX) on the New England Mud Patch is the extent to which gradients exist in geoacoustic properties (sound speed and density) over the upper mud layer. This paper applies Bayesian inversion and uncertainty quantification to modal dispersion data, resolved by warping analysis, to consider whether mud-layer gradients are required to fit the data. The inversion is based on a hybrid seabed-model parameterized in terms of an upper sediment layer with a general representation of smooth, continuous gradients in geoacoustic properties based on Bernstein-polynomial basis functions, above an unknown number of discrete layers formulated trans-dimensionally. The inversion results are compared to those from other acoustic data sets collected in the region as well as to nearby core measurements.

Estimation of geoacoustic parameters using machine learning techniques

Modal dispersion of broadband acoustic data has been used extensively to estimate the geoacoustic parameters using perturbation techniques (Rajan et al., 1987), non-linear inversion techniques (Potty et al., 2000) and Bayesian inference (Warner et al., 2015). The Airy phase region corresponding the minimum group velocity of the normal modes are extremely sensitive to bottom properties in comparison with properties of the water column. This talk will explore the sensitivity of the group speed minima and the associated frequencies of the acoustic normal modes to bottom parameters. The group speed minima and the associated frequency data will be used to train a machine-learning algorithm. Synthetic data will be generated for various bottoms and it will be used as the training pool. Data collected from New England Bight, East China Sea, and New England Mud patch will be used to test the algorithm. The value added to the a priori bottom parameter information using the algorithm will be discussed in the context of computational cost associated with the algorithm. [Work supported by the Office of Naval Research.]

Geoacoustic Inversion for a New England Mud Patch Sediment Using the Silt-Suspension Theory of Marine Mud

This article provides an application of the silt-suspension theory to a Bayesian-inference inversion for the geo-acoustic parameters in marine mud. The theory, with consequences that have been developed recently, postulates a suspension of water and clay mineral card-houses that supports moderately dilute concentrations of silt particles. The approach is an example of a physically based model inversion, in which parameters representing physical mud-layer properties are obtained by inversion and used to produce estimates of geoacoustic properties, including their frequency dependence. The acoustic data are from a combustive source signal propagated along a track, located over several meters of fine-grained mud in the New England Mud Patch, to a single hydrophone on a receiver array during the 2017 Seabed Characterization Experiment. Data extracted from a nearby piston core inform the physical modeling, with selections of inversion parameters guided by both sensitivity analyses and bounds from archival and core measurements. Results show the feasibility of this inversion approach. The estimates of mud density and sound speed are close to values obtained independently. The frequency dependence of attenuation is estimated over the full low-frequency source band and has an approximate power exponent of 1.72.

Short-Range Signatures of Explosive Sounds in Shallow Water Used for Seabed Characterization

Small explosions were used as sound sources in the Seabed Characterization Experiment conducted in spring 2017 in the New England Mud Patch, a shallow-water region with a depth of approximately 75 m. The sources were U.S. Navy Signal Underwater Sound (SUS) Mk 64 charges, which contained 31.18 g of the explosive 2,4,6-Trinitrophenylmethylnitramine, commonly referred to as Tetryl. Source recordings were obtained by two hydrophones deployed from the same ship that deployed the SUS. The recordings were analyzed for bubble period, energy spectral density, and the variability of these parameters, and compared to previous results from the literature, including the prediction of a historic spectral model, and a new semiempirical time-domain model assembled using measured data from the literature. The new model describes the source level measurements in the 25–275-Hz band and in the 400-Hz octave band to within 0.5 dB, and agrees with similar measurements from the literature to within 0.6 dB. The standard deviation of the band-limited source levels was found to be about 1 dB, some of which is ascribed to uncertainty and variation in the source-to-receiver distance. The observed source level variation is similar to previously reported values.

Estimation of the Geoacoustic Properties of the New England Mud Patch From the Vertical Coherence of the Ambient Noise in the Water Column

The autonomous passive-acoustic lander Deep Sound was deployed at five locations during the Office of Naval Research (ONR)-supported Seabed Characterization Experiment, a multi-institutional field effort held at the New England Mud Patch, where the seabed is known to consist of a thick layer of silt and clay overlying a medium and coarse sand. The five deployments of Deep Sound were up to 9 h long, during which time ambient noise data, taken over an acoustic bandwidth of 5 Hz–30 kHz, were collected on four hydrophones arranged in an inverted “T” shape. Local temperature and conductivity were also recorded continuously at each location. A wave number integral model of wind-driven noise in a fluid waveguide over a two-layered elastic seabed was used to calculate the dependence of the vertical noise coherence on the geoacoustic properties in the overlying silt and clay layer, as well as the subbottom sand half-space. The modeled noise coherence was fitted to the data over the band 100 Hz–12 kHz, returning the compressional- and shear-wave speeds and densities of both layers and the thickness of the top layer.

Seasonal variations in the surficial sediment grain size in the East China Sea continental shelf and their implications for terrigenous sediment transport

This study discusses the seasonal variations in surficial sediment grain size and their implications for sediment transport driven by the hydrodynamics in the East China Sea, based on grain size analysis of surficial sediments and salinity data collected from the East China Sea continental shelf during two cruises in 2011. The results show that the grain size distribution on the East China Sea continental shelf presents apparent spatial and seasonal variations. The mud area exhibits more variation than the sand area, which indicates that the sediment supply is the main factor controlling the surficial sediment grain size distribution on the East China Sea continental shelf. The grain size near the Yangtze River Estuary is relatively coarse in summer, because of the inherited characteristics from the previous winter, whereas it is much finer in autumn, due to the input of fine-grained materials during the flood period of the Yangtze River. A high fine-grained sediment concentration was found along the Zhejiang-Fujian coast in autumn when the Zhejiang-Fujian Coastal Current forms. The sediments in the northeast of the study region (the mud patch southwest of Cheju Island) are much coarser, and the fine-grained area is narrower in autumn than in summer because of seasonal variations in the Yellow Sea Coastal Current and the frequency of storms. Simultaneously, the tongue-shaped fine-grained cross shelf front located along the Zhejiang-Fujian coast occurs farther to the south in autumn than in summer. However, the surficial grain size distribution patterns on the middle and outer shelf are similar in the two seasons.

Synoptic variations of residual currents in the Huanghe (Yellow River)-derived distal mud patch off the Shandong Peninsula: Implications for long-term sediment transport

Huanghe (Yellow River) is one of the largest contributors of global terrestrial materials to the sea. A large amount of the Huanghe-derived sediment are responsible for forming not only the nearshore delta depositional system, but also a distal mud patch off the Shandong Peninsula, some 350 km east of the present-day river mouth. To better understand the spatial and temporal variations of sediment dynamics in the distal mud area, two field surveys employing the phase-averaging method (PAM) were conducted in September 2017 and January 2018, respectively along a 108-km East-West transect across the mud patch. The residual currents derived from the PAM surveys were featured by the southward current in the nearshore area and the northward current in the offshore region in summer, while a unidirectional current transported from south to north across the entire transit line in winter. These patterns, which deviate from the traditional understandings of circulations in magnitude and even in direction, are believed to result from episodic wind reversals and associated strengthening and weakening sea level pressure gradients. The northerly wind episodes in summer, works hand-in-hand with sea-level tilt by preceding southerly wind, to enhance the water exchange between the Bohai Sea and the Yellow Sea by intensifying both the southward flow along the coast and the northward flow of the South Yellow Sea water further offshore. In winter, episodic southerly wind causes relaxation or even reversal of the prevailing winter northerly wind that created the northward sea-level tilt in the Bohai Sea and the Yellow Sea, effectively enhances the South Yellow Sea water transport to the North Yellow Sea and the Bohai Sea. Consequently, a considerable amount of sediment previously exported out of the Bohai Sea to the mud patch actually return with these backflows. Since the synoptic time scale oscillations in the Bohai and Yellow seas are considerably frequent throughout summer and winter, the cumulative effects on the long-term sediment transport need to be further investigated. The observations in this study provide a base for new understanding of how episodic weather events impact the regional circulation and sediment dynamics on the Huanghe-derived distal mud patch.

Estimates of Low-Frequency Sound Speed and Attenuation in a Surface Mud Layer Using Low-Order Modes

Whereas there have been numerous theoretical and experimental studies on the properties of marine granular sands, there are significantly fewer studies on sediments classified as muds. The validity of geoacoustic models for muddy sediments has not been successfully tested due to the lack of inverted low-frequency sound speed and attenuation values. The geoacoustic properties of a surface fine-grained mud layer, overlaying three sand transition layers, and a half-space basement within the New England Mud Patch, were studied using explosive signals from long-range along-shelf sound propagation tracks. The sound-speed profile of the mud layer in the low-frequency band (100-500 Hz) was estimated using acoustic normal mode characteristics including the mode shapes and the modal dispersion curves of low-order modes, which mainly propagated in the water column and the surface mud layer. The ambiguity of sound speed at the top of the mud layer and sound-speed gradient was approximately removed. It was found that the resultant sound-speed ratio at the water-sediment interface was close to unity and the sound-speed gradient was 1.8 1/s with a standard deviation of 1.0 1/s. The attenuation in the mud layer was inverted using the attenuation coefficient of the first mode extracted from explosive signals at three source locations. The estimated attenuation at 150 Hz had a mean of 0.006 dB/m and a standard deviation of 0.003 dB/m.

In Situ Measurements of Compressional Wave Speed During Gravity Coring Operations in the New England Mud Patch

This paper presents measurements of sediment sound-speed profiles measured in situ using the acoustic coring system (ACS). The reported measurements were obtained from seven gravity cores collected in the New England Mud Patch. The ACS uses two sets of transducers mounted below the penetrating tip of a sediment corer to make in situ measurements of geoacoustic properties as the corer penetrates the seabed. The in situ sound-speed profiles are interpreted in the context of stratigraphic layering measured by a seismic survey, and the measured sound-speed profiles are consistent with the geophysical description. The in situ sound-speed profiles measured by the ACS were compared to conventional measurements of sound speed on the recovered cores using the multisensor core logger (MSCL). The MSCL data displayed both random and systematic errors that were attributed to disturbance from the coring process and/or the handling of the sediments after collection. Finally, using porosity and grain size distributions measured from discrete samples of the cored sediments, the in situ sound-speed measurements were compared to empirical regressions based on independent data sets.

Broadband Waveform Geoacoustic Inversions With Absolute Travel Time

Numerical and experimental studies were conducted to investigate the advantage of utilizing absolute travel time information in Bayesian geoacoustic inversions of broadband acoustic data. It is shown that inversions using absolute travel time can yield smaller uncertainties compared to inversions using relative arrival time and maximum-likelihood estimates for clock time synchronization. Experimental data collected in the Seabed Characterization Experiment on the New England Mud Patch in the Middle Atlantic Bight were used for real data demonstration, and it is shown that inversions using relative arrival times have greater uncertainty in estimating source distance, which consequently affects the overall posterior probability distribution of inverted parameters. Numerical study enables investigation of performance dependence on the signal-to-noise ratio, and it is found that absolute travel time information may have more profound advantages when the signal-to-noise ratio is low.